This invention relates generally to the field of beverage and food preparation, and, more particularly, to an apparatus that aids in the proper heating of beverages and liquid-foods in a microwave oven. The preferred embodiment is oriented primarily towards beverage preparation in a microwave oven, but the invention described herein could be equally valid when applied to the preparation of foods in a microwave oven.
Microwave ovens are in common use today to prepare hot beverages. Microwave ovens cause the water molecules in the beverage to oscillate and thereby generate heat. One common use of today's microwave ovens is to heat small amounts of water for use in making coffee, tea, or other hot beverages, or for fluid-based foods such as soup and jello. It is quite common for water to be poured into a drinking cup, mug, or bowl, placed in the microwave oven, and brought to a high temperature. Coffee, tea, other beverage matter, or soup mix is then added to this hot water (or the hot water is poured into another food item) and the user drinks directly from the cup, mug, or bowl. It is generally known that the water should be at or very near boiling for maximum flavor extraction of the applied beverage or food ingredients.
Boiling point indication. Microwave ovens often vary in the amount of power they generate, and thus, the degree to which they are able to heat water. One microwave oven may take several minutes to boil a given amount of water that another microwave oven might boil in much less (or more) time. Boiling water in a microwave is thus somewhat a matter of trial and error, whereby the user heats a cup of water for a period of time, tests the temperature by sight (to see if it is bubbling or steaming), by touch, or by taste, and then repeats the heating process if a complete boil has not been reached.
It is also common for people to forget they are heating a vessel of water in a microwave. The heated water cools during this period of forgetfulness. Upon remembering they were preparing a beverage, the user returns to the microwave and re-heats the water. This repetitive re-heating action effectively reduces the flavorful potential of the water by removing dissolved oxygen. Furthermore, this reheating action makes the dangerous act of “superheating” the water more possible (These effects of reheating are both explained in detail later in this section).
Another factor that can vary the amount of time it takes to boil water is simply that the amount of water to be heated can vary. It is quite common for users of microwaves to use a variety of cups, mugs, and bowls, each with different capacities. These varying capacities make it likely that the user will be heating varying amounts of water. Furthermore, it is also quite possible for users to use the same vessel, but having filled it with slightly different amounts of water. Thus, while a user may be using the same microwave oven, the variation in the amount of water being heated at any given instance results in a variation in the amount of time required to boil that water. This variation in time thus, once again, makes boiling water in a microwave an imprecise and inefficient act.
Thus, there exists a need for a device that will conveniently and clearly indicate when a liquid heated in a microwave has reached its boiling point, regardless of the power output of the microwave or the amount of liquid being heated.
Superheating. Microwaves are prone to a scientific phenomenon known as “superheating.” Superheating refers to heating of a liquid above its boiling point. Microwaves heat water by being directly absorbed by the water. The microwave energy converts to tumbling of the water molecules, which makes the water hot. When the water boils, molecules of the water rapidly pass from the bulk liquid into vapor bubbles. To make a bubble in the first place, a lot of water has to be moved out of the way, because vapor is much less dense than liquid. The new vapor must push outward against both the internal pressure of the liquid (e.g. water) and the vapor/liquid interface, which provides additional force to collapse the bubble. So, it takes energy and the right combination of molecular motions or the existence of a seed air pocket to form a bubble that can act as a nucleus for boiling. Alternately, an existing pocket of air created by the presence of an imperfection in the vessel surface or by a piece of lint or dust can serve as a suitable seed for a boiling bubble.
Superheating is not a problem with conventional, stove-top heating of liquids. When water is heated in a pot on the stove, the hottest region of the liquid is that right next to the bottom of the pot wall. In addition, the pot walls usually contain many small scratches that hold small bubbles of air, which can act as starters for the boiling process. With the hottest liquid right next to the bubbles that start the boiling, conditions are ideal for boiling to start when water is heated on a stove.
Heating water in vessels commonly-used with microwaves makes boiling more difficult for two reasons. The first is that glasses and glazed ceramic (common materials for cups or other vessels used in microwaves) contain fewer surface scratches than metal pots. This lack of scratches and imperfections provides fewer loci for starter bubbles.
The second reason that water is so commonly superheated in microwaves is that the hottest portion of the liquid is not right next to the vessel surface and its little air bubbles. When water in the middle of the cup is heated to above its boiling point, it is very hard for it to create the bubbles necessary for boiling.
In other words, you will not have boiling water if there are no sites for the vapor (within the liquid) to nucleate (grow) from. Good “nucleation sites” are scratches, irregularities, and other imperfections in the container holding the liquid. Thus, water doesn't always boil when it is heated above its normal boiling temperature (100° C. or 212° F.). Something has to trigger the formation of steam bubbles, a process known as “nucleation.” If there is no nucleation of steam bubbles, there will be no boiling and therefore no effective limit to how hot the water can become. If a microwave heats a cup of water beyond the boiling point, the water is said to be “superheated.” In practice, a person can heat a cup or mug of water for several minutes and then visually inspect the water to see if it is boiling. If it does not appear to be bubbling and boiling, the user is inclined to heat the water further. In fact, the water may already be at (or above) its boiling point—it just isn't bubbling because there are no nucleation sites. The user then (mistakenly) concludes the water isn't boiling and heats it further, thereby “superheating” it.
Furthermore, the act of heating water to boiling and then allowing it to cool also makes superheating more likely upon reheating. This is because the initial boiling may remove some of the pieces of lint, dust or other catalysts that create seed bubbles for boiling to occur. As stated earlier, it is common for microwave users to begin heating a vessel of water only to forget they are doing so. Upon remembering, the user reheats the water, thereby increasing the relative likelihood of superheating that container of water.
Superheated water is extremely dangerous—all it takes is some trigger to provide a nucleation site to create the first bubble. This trigger could be a spoon inserted into the water, a granule of coffee powder, a teabag, etc., or even simply jarring the vessel. The instant the superheated water receives the nucleation trigger, an explosive eruption of the liquid can occur (as the excess energy is rapidly converted to vapor). The Food and Drug Administration of the United States has received reports of serious burns or scalding injuries around people's hands and faces as a result of extremely hot water erupting out of a cup after it has been superheated in a microwave.
“Superheating” is a common scientific phenomenon that sometimes occurs in the boiling of any liquid. For this reason, it is common practice in laboratory situations to put “boiling chips” or “boiling stones”—pieces of pumice, rock, or other material that provide nucleation sites for boiling to occur—into containers of liquid being boiled.
A related problem is that of “over-boiling” (continuing to boil water after it has reached its boiling point). Over-boiling drives out oxygen dissolved in the water (“cuts” the water) and tends to impart a flat taste to a beverage. It therefore becomes desirable to know just when the water is brought to a boil so that the heating process can be stopped.
Thus, there exists a need to prevent dangerous superheating of water that can occur in microwave ovens. There also exists a related need to prevent over-boiling to maximize the flavorful potential of the beverage liquid.